1. Field of the Invention
The present invention relates to a torque sensor having a housing which has a light source and a measuring shaft which has a signal generator arrangement associated with a rotation angle sensing device with a processor.
2. Description of the Related Art
A torque sensor of this type is disclosed by EP 0 555 987 B1. The known torque sensor is used in a power-assisted vehicle steering system. It is arranged between a steering wheel and a steering gearbox. The wheels are acted on by a motor which is driven in such a way that the torque determined by the torque sensor becomes as small as possible. The measuring shaft is in this case designed as a torsion shaft, that is to say it is rotated or twisted somewhat when a force is applied to the steering wheel. The signal generator arrangement operates optically. It has two diaphragm disks which, when a torque occurs, are rotated in relation to each other. A receiver arrangement receives a greater or lesser luminous flux through the diaphragm arrangement, and accordingly outputs a greater or lesser signal current, from which the torque acting on the measuring shaft can then be calculated.
Another torque sensor is disclosed by EP 0 765 795 A2. This torque sensor also determines the relative position of the two ends of the measuring shaft, specifically by using transfer devices which induce different voltages in different coils.
Problems arise in such torque sensors when the torque is to be determined on a rotable shaft. However, in the case of a steering shaft this is usually the case. The steering wheel has to describe a number of revolutions between the two extreme positions of the steered wheels.
For this reason, in the first-mentioned printed document, only the diaphragm arrangement can be rotated. The light source and the receiver arrangement are fixed to the housing. This requires a relatively complicated construction. In order to transmit the electrical signals with the aid of transfer devices, relatively high amplitudes are necessary, and it likewise necessitates a complicated construction.
One object of the invention is based on being able to determine a torque in a simple way, even on rotating shafts. Other objects and advantages of the present invention will become apparent from the following summary and detailed description of the presently preferred embodiments.
In a torque sensor of the type cited at the beginning, this object is achieved by the rotation angle sensing device being arranged to rotate relative to the housing, together with the measuring shaft, and by the processing device having an opto-electric power supply.
This configuration permits the entire rotation angle sensing device to be arranged to be stationary in relation to the measuring shaft. Consequently, the entire evaluation of the rotation angle sensing can take place directly on the measuring shaft. The risk that interference will be introduced into the signals is relatively low. The electrical power which is necessary to operate the processing device is supplied on an optical path. The power supply device converts the incident luminous flux into an electrical voltage and, if appropriate, conditions this voltage. Since no relatively large amounts of electrical power are usually required for the signal processing in the rotation angle sensing device, the optical power, transmitted with the aid of light, is entirely sufficient.
One end of the measuring shaft is preferably connected to a first carrier, which has an optical detector arrangement, and the other end of the measuring shaft is connected to a second carrier, which has a diaphragm arrangement, the second carrier being arranged between the first carrier and the light source. If a torque is applied, then the measuring shaft is twisted somewhat. Consequently, the two carriers are displaced in relation to each other. The diaphragm arrangement, which is arranged between the light source and the first carrier, accordingly shades certain regions of the optical detector arrangement and exposes others. In this way, a differential signal can be obtained which permits an improved determination of the relative displacement of the two carriers in relation to each other (in the circumferential direction). In this case, it is particularly advantageous that the same light source which is used to obtain the position signal can also be used to supply the electrical power. This simplifies the construction.
The second carrier preferably has a photovoltaic arrangement on its side facing the light source. A photovoltaic arrangement may be formed, for example, by so-called xe2x80x9csolar cellsxe2x80x9d, which convert incident light directly into electrical power. Accordingly, no further conversion steps, for example via a heat/force coupling, are required. The solar cells may be formed, for example, by photoelectric semiconductors.
The spectrum of the light source is advantageously matched to the sensitivity of the photovoltaic arrangement. In this way, a high efficiency is achieved. The diaphragm arrangement is preferably formed by at least one aperture in the photovoltaic arrangement and its carrier. Consequently, it is possible to utilize the entire area of the carrier to accommodate the corresponding solar cells. The larger the area which is available to receive light, the more power can be transmitted to the photovoltaic arrangement and therefore to the power supply device.
The processing device preferably has a transmitter which interacts with a receiver in the housing via a wire-free link. The measuring shaft can therefore rotate freely, without any lines needing to be connected to the housing. However, there is a line between the two carriers. Since the relative movement between these two carriers is very small and moves within the range of one degree, it is still possible to operate here with a flexible line. Otherwise, the multiple rotation of the measuring shaft is not impeded by lines.
The wire-free link is preferably designed as an optical transmission link. An optical transmission link has the advantage that it is relatively insensitive to interference which can be introduced from outside. However, such immunity to interference should certainly be provided in the case of a motor vehicle.
The optical transmission link is preferably arranged on that side of the first carrier which faces away from the light source. The optical transmission link is therefore shaded from the light source. This permits the optical transmission link to be provided with a relatively high sensitivity.
The optical transmission link advantageously has a number of receiver elements distributed around the shaft. It is then possible to manage with one or, in any case, a few transmitters, and nevertheless to achieve a reliable transmission of the torque signal values for each rotation angle of the measuring shaft.
In this case, it is particularly preferred for the transmitter to output a frequency-analog, pulse-width modulated or carrier-frequency signal or another amplitude-independent signal. A signal of this type, whose information is carried not in the amplitude but in other variables, can be transmitted with high reliability, the respective distance between the transmitter and the nearest receiver element being unimportant. The influence of interfering light, which may arise, for example, from the light source, can also be kept small.
The light source preferably operates in cycles. Although this reduces the power which can be transmitted, this can be tolerated if the power supply device is designed to be appropriately large. However, the advantage which is achieved in this way is that interfering variables can be filtered out with relatively high reliability. The signal detected by the optical detector arrangement, or the signals determined, generally contain a useful component which depends on the exposure or shading by the diaphragm arrangement, and an interference component. The interference component can be caused by stray light, by parasitic currents or by aging-induced changes in the components. The interference component is independent of the exposure or shading of the photodetector by the diaphragm arrangement. It therefore remains constant even when the light source changes its light intensity. If, therefore, the signals from the optical detector are compared with one another in different phases of a cycle, it is possible to eliminate the corresponding interference components. In a particularly simply configured embodiment, this is the case if the light source transmits no light in one portion of each period. In this case, only the interference signal is produced in this portion, and this can then be removed again in a following portion from the combined interference and useful signals.
In a particularly preferred configuration, provision is made for the measuring shaft to be surrounded by a hollow shaft which bears one of the two carriers, the other carrier being of circular-segment design and projecting through a corresponding slot in the wall of the hollow shaft. This has the advantage that the hollow shaft can be supported on both sides of the measuring shaft. The movement of the hollow shaft in relation to the measuring shaft is then restricted to pure rotation. Consequently, the result is no other displacements of the two carriers in relation to each other. Since one carrier is of circular-segment design, it is able to project through the slot in the hollow shaft without excessively weakening the hollow shaft.
In this case, it is particularly preferred for a pin to be led through that end of the measuring shaft which is not connected to the hollowing shaft, said pin also being plugged into the hollow shaft and being retained in one of the two parts, measuring shaft and hollow shaft, and the opening in the other of the two parts being larger than the cross section of the pin. The pin therefore permits a relative rotation of the hollow shaft in relation to the measuring shaft. The opening only has to be sufficiently large that the pin does not make contact with that part containing the opening at the maximum permissible torques. However, should the torque become greater, then the pin provides security against this greater torque also acting on the measuring shaft. The pin therefore forms an overload safeguard. The designation xe2x80x9cpinxe2x80x9d is in this case a contraction for an element that satisfies the function described. It may also actually be a rod, for example a cylindrical one. However, it is also possible for a type of ring gear arrangement to be provided, in which the measuring shaft and the sleeve each have ring gears which interengage and are provided with appropriate play in order in this way to absorb the acting forces over the entire circumference.